The invention relates to the production of electronic circuits, more particularly to V compositions and processes for the production of passive components such as resistors, capacitors and inductors for rigid and flexible printed circuits on polymer-based substrates using print-and-heat technology
Electric resistors, capacitors and inductors are now almost uniformly produced with oxide ceramic active elements, even for discrete components, which are assembled to circuits by soldering. Examples of such materials include: silver-palladium oxide resistors; ruthenium oxide resistors; barium ruthenate resistors; barium titanate ferroelectric capacitors and piezoelectric elements; lead titanateizirconate ferroelectrics; spinel-type ferrites such as magnetite; hexaferrites; and gamet-type ferrites.
Passive electronic components such as capacitors and resistors have been prepared on substrates by printing xe2x80x9cDick filmxe2x80x9d inks on ceramic substrates to make xe2x80x9chybrid circuitsxe2x80x9d for many years. Thick film inks and pastes consist of a suspension of particulate material with the desired electrical properties in an organic vehicle. The vehicle consists of a viscous liquid such as a-terpinol with a resin such as ethyl cellulose for viscosity modifications and minor additions of other constituents to improve printability. Glass frit is added to the mixture to improve bondability to the substrate. A hybrid circuit consists of metallic circuit traces, screen printed on ceramic substrates and fired at temperatures in the neighborhood of 1000xc2x0 C. to sinter the metal and fuse the glass to bond the metal and the ceramic. During the firing process, the organic constituents of the inks volatilize and disappear. Their function is strictly to facilitate printing. Silicon integrated circuits are usually bonded to the traces creating a hybrid between the silicon circuitry and the ceramic circuitry.
Thick film resistor inks can be used to print resistors rather than bonding discrete components to the circuit. Silver-palladium mixtures, which can be oxidized to silver-palladium metal and semiconducting palladium oxide, have been widely used as resistive films as described by Larry, J. R; Rosenberg, R. M.; Uhier, R. O.; in Trans IEEE, CHMT-3, (2), 211-225, 1980. In recent years semiconducting ruthenium oxide compositions have become dominant.
Thick film capacitors have been prepared by printing a metallic plate, printing a layer of thick film ferroelectric dielectric, such as barium titanate, and printing another metallic plate to complete the capacitor. At high temperature the barium titanate sinters into monolithic ceramic and grain growth occurs, improving its dielectric properties.
Ferrite inductive cores for transformers, chokes and antennas can be printed with inks containing ferrite particles, which sinter to monolithic ceramics at the high processing temperatures of thick film technology.
xe2x80x9cPolymer Thick Filmxe2x80x9d materials are widely used to print silver conductive traces on polymer substrates such as membrane touch switches and keyboards. These xe2x80x9cpolymer thick filmxe2x80x9d materials are similar to conventional thick film inks, except the organic vehicle is an epoxy resin which is not volatilized and remains as an integral part of the finished circuit component. The electrical conductivity of these materials is less than one tenth that of conventional thick film conductors because the conductivity depends on adventitious contact between the individual silver flakes in the epoxy matrix rather than on the monolithic sintered structure achieved in high temperature thick film processing. There have been attempts to create polymer thick film capacitor dielectrics and inductive cores, however; these problems are even more severe in these materials which depend for their electrical properties on the creation of void-free recrystalized structures. Since the particles are isolated by organic matrix, even though each particle in the polymer thick film ink may have a high dielectric constant or magnetic permeability, the properties of the deposit are closer to those of the matrix than to those of the particles.
A new class of compositions now known as Parmod(trademark) have been used in a low temperature process for producing high conductivity electrical conductors on temperature sensitive substrates. Examples of such Parmod(trademark) compositions are disclosed in U.S. Pat. No. 5,882,722 (the disclosures of which are here by incorporated in full by reference hereto) and U.S. application Ser. Nos. 09/034,069; 09/484,882; and 09/367,783 (the disclosures of which are here by incorporated in full by reference hereto). Such Parmod(trademark) compositions contain a Reactive Organic Medium (ROM) and metal flakes and/or metal powders. The ROM consists of either a Metallo-Organic Decomposition (MOD) compound or an organic reagent, which can form such a MOD compound upon heating in the presence of the metal constituents. The ingredients are blended together with rheology modifying organic vehicles well known in the art, if necessary, to produce printing inks or pastes. These inks can be printed on a temperature sensitive substrate and cured to well-consolidated, well-bonded electrical conductors at a temperature low enough so that the substrate is not damaged. The curing process occurs at temperatures far below those used for conventional sintering of thick film inks and pastes. Parmod(trademark) compositions can also be formulated into liquid toners capable of being printed using electrostatic printing methods. Examples of such toners are disclosed in U.S. application Ser. No. 09/369,571 (the disclosure of which is hereby incorporated in full by reference hereto).
A major application of these Parmod(trademark) materials is producing printed circuits by a fully additive process in which the Parmod(trademark) mixtures are printed on the polymer-based circuit substrate and cured to well-bonded, pure metal conductors by heating to a temperature in a range that the substrate can stand, for example, about 200-350xc2x0 C. This process eliminates the steps of laminating copper to the substrate, laminating a photoresist to the copper, exposing the resist through a mask, developing the resist, etching away the unwanted copper and stripping the resist that comprise conventional subtractive processing.
The Parmod(trademark) process and materials make use of the unexpected property that mixtures of metal powders with metal-containing reactive organic materials can be decomposed to well-consolidated metal traces in which the metal particles appear to be xe2x80x9cchemically weldedxe2x80x9d together into a continuous metallic phase. This chemical process takes place at temperatures that are hundreds of degrees lower than those required for conventional metallic sintering, and in times that can be measured in seconds rather than minutes or hours. The result is the ability to produce the equivalent of xe2x80x9cthick filmxe2x80x9d circuits which are produced by screen printing sinterable metallic pastes on ceramic substrates and firing at temperatures of 500xc2x0 C. to more than 1000xc2x0 C., but to do it on much less expensive polymer-based substrates and cure them in much less time at much lower temperatures. In the case of metals this results in a porous but continuous metal trace which has a density approximately half that of bulk metal and an electrical conductivity per unit mass which is also approximately half that of the bulk metal. The printed Parmod(trademark) conductors are made up of continuous well-bonded metal rather than individual particles, which are in adventitious contact with each other. xe2x80x9cpolymer thick film materialsxe2x80x9d, which consist of silver flakes in an epoxy continuous phase, have an electrical conductivity based on mass of approximately one tenth that of Parmod(trademark) .
It would be desirable to be able to print resistors, capacitors and inductors on to low temperature substrates and fire them prior to or simultaneously with the metallic conductors at the low temperatures achievable with Parmod(trademark) technology
It is an objective of this invention to provide inks and pastes which will decompose to form electrical components such as resistors, capacitors and inductors at a low enough temperature to be used on conventional polymer-based printed circuit boards like polymer thick film materials, but which have the good electrical properties of conventional high temperature thick film materials.
Parmod(trademark) technology is used to create continuous oxide phases from printable inks and pastes by mixing high performance powders with a reactive organic medium, which can weld them together into a continuous phase at low temperature by decomposing into the same chemical composition as the powder phase.
The Parmod(trademark) mixtures of this invention function by deposition of material from the decomposition of the MOD compound which xe2x80x9cchemically weldsxe2x80x9d the powder constituents of the mixture together into a monolithic solid.
The resulting passive components can be produced and connected as part of the printed circuit fabrication process, rather than being separately produced and assembled onto the printed circuit by soldering.